The present invention relates to a diagnosis system for internal combustion engines, and more particularly to a system for diagnosing failure of an air-flow meter system which operates to measure the volume of intake air of an engine mounted on a motor vehicle.
A fuel-injection system of the type which is provided with a computer operable to compute the volume of intake air from an output signal from the air-flow meter in order to produce output signals for driving solenoid-operated injection valves is known. If a fault occurs, such as a disconnection of wires for the air-flow meter or disengagement of a connector of the air-flow meter from a terminal of a unit, control of the air-fuel ratio of injected fuel can not be done. Accordingly, such a fault is immediately detected to operate a fail-safe system so as to prevent stalling of the engine. A conventional detecting means comprises a potentiometer operated by a flap in an intake passage and a smoothing capacitor. When the voltage at the capacitor decreases to a low level by the fault, the fail-safe system operates. Since the voltage at the capacitor fluctuates during the normal operation of the engine, the voltage, at which the fail-safe system becomes operative, is set to a considerably low level in order to prevent misjudging of the detection. Therefore, a relatively long time elapses before the voltage decreases to the preset low level at the fault, so that the engine stalls.